The invention relates to the field of combustion chambers of turbine engines and, more specifically, to the configuration of the dilution air intake openings and the cooling air passage perforations formed in the walls of the flame tube or in any combustion chamber wall element.
FIG. 1B shows a view in axial section of a turbomachine combustion chamber 1 according to the prior art, as described in patent document EP-A-0 743 490 in the name of the applicant.
The combustion chamber 1 is formed by two concentric tubular side walls 3, constituting a flame tube (extending in the longitudinal direction L-L of the chamber, parallel here to the axis X-X of the turbomachine). The chamber is closed at one end, the upstream end M, by an annular end wall 4 at which are located fuel injectors 6 and oxidizer air inlets 7, the combustion of the fuel and oxidizer generating a stream of combustion gases. The chamber is terminated at the other end, the downstream end V, by an annular orifice 5 for expelling the stream G of burnt gases destined for the rotating gas turbine of the turbomachine.
As illustrated in FIG. 1B, dilution openings 8 or holes are formed in the side walls 3 of the chamber 1 so that an additional stream A of fresh air can be mixed into the stream G of combustion gases which propagates toward the downstream end V of the chamber 1. This additional fresh air A serves to dilute the burning gases G, to reduce their temperature, to cool the walls and to increase the proportion of air in the gas mixture. This is done in an attempt to optimize the stoichiometry of the oxidizer air/fuel mixture, to burn the unburnt residues and to reduce emissions of NOx—nitrogen oxides—, with the aim of improving the combustion of the gas mixture G (especially by prolonging, over the entire extent of the chamber, the combustion of the initially too rich mixture upon ignition).
The dilution air intake openings 8 pierced in the side walls 3 are arranged along the circumference of the tubular walls at a central axial position between the end wall M and the orifice 5 of the chamber 1.
Various techniques are known in the prior art for forming the dilution openings 8.
As illustrated in views 1A and 1C, there are dilution openings 8′ known as “square-edge holes”. The opening 8′ is obtained by simple normal piercing (with a drill or by cutting with a punch) of a cylindrical bore with straight edges perpendicular to the wall 3 of the chamber 1. The opening 8′ can also be produced by laser.
These dilution openings 8′ with straight edges according to the prior art have the disadvantages of not allowing good intake of the dilution air stream D and of not providing good efficiency. The compressed fresh air stream A which flows in the bypass duct 2 around the combustion chamber 1 and which sweeps along the side walls 3 of the chamber is suddenly deviated at a right angle D to pass along the axis T-T of the opening 8′.
There is another known technique for producing dilution openings 8, as illustrated in views 1B and 1D, in which the openings 8 have “bent-over edges”, that is to say edges folded toward the inside of the chamber 1 and observing a certain degree of curvature (edges having “radiused” or rounded regions), giving them a crater shape.
These dilution openings 8 with “bent-over edges” have the disadvantages of being exposed to the incidence of the stream of burning gases G, thus causing the appearance of hot spots and sometimes burn regions on the crest of the “crater” formed by the edge of the opening 8, and especially in the wake region downstream of the opening, because of the vortex S caused by the incidence of the longitudinal stream of burning gas G on the crest of the edge 8 which projects transversely with respect to the inside of the chamber 1.
Moreover, beside the dilution openings 8′ (commonly known as dilution holes/apertures), which have relatively large dimensions, the walls 3 of the chamber 1 comprise perforations 9 having tiny dimensions. These microperforations are distributed over the entirety of these metal walls 3, preferably being concentrated in the vicinity of the dilution openings 8′. These perforations (commonly known as impingement holes) serve for the injection of microstreams of air whose primary function is to cool the metal mass of the side walls 3 to enable them to withstand the very high temperatures (more than 1000° C.) of the burning gases G in the combustion chamber 1. A distinction should be drawn here between these microperforations for injecting cooling air, referred to here as cooling perforations, and the relatively large dilution air intake openings, referred to here as dilution openings.
Another disadvantage of the dilution openings 8′ with “bent-over edges” is that the curvature of the folded edges does not allow cooling perforations to be pierced in the immediate vicinity of the opening 8 and specifically in the regions exposed to the formation of hot spots or burns, which would require effective cooling. The deformation of the edges of the dilution opening prevents the perforations from being brought up close to the edges without adversely affecting them.
The aim of the invention is to overcome the disadvantages of the current solutions and to produce a combustion chamber provided with dilution openings for optimizing the intake of the air stream while as far as possible preventing turbulence and the formation of hot spots that are detrimental to the thermomechanical integrity of the combustion chamber and to its service life.